The Claus process is widely used, notably in refineries (after hydrodesulfurization or catalytic cracking units) and for processing of natural gas, to recover elemental sulfur from gaseous feeds containing hydrogen sulfide. However, the fumes produced by Claus plants contain, even after several catalytic stages, appreciable amounts of acid gases. It is then necessary to process these Claus plant effluents (tail gas) to remove most of the toxic compounds so as to abide by antipollution standards. Since these standards become increasingly severe, it is essential to permanently improve the existing technology.
It is for example well-known to recover about 95% by weight of the sulfur present from a Claus plant.
Processing this Claus plant effluent with a Clauspol plant allows for example to reach 99.8% by weight of solvent recovered, from the exothermic Claus reaction:2H2S+SO23S+2H2O  (reaction 1)
Such processing requires a reaction medium consisting of an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt of an organic acid. Contacting the gas to be processed and the organic solvent containing the catalyst can be carried out countercurrent or cocurrent, in a vertical or horizontal gas-liquid contactor reactor whose temperature is controlled by passage of the solvent that has been extracted at one end of the contactor reactor by a circulation pump into a heat exchanger so as to favour the highest sulfur conversion coefficient while preventing formation of solid sulfur. It is well-known that, in this type of plant, the solvent that has a limited capacity for dissolving elemental sulfur becomes loaded with free liquid elemental sulfur that can be separated from the solvent by simple decantation. This liquid sulfur—solvent decantation is carrier out in a liquid-liquid decantation zone that can be situated at the bottom of the contactor reactor. The sulfur is thus recovered in liquid form.
Operation of such a plant is for example described in one of the following reference books:
Y. BARTHEL, H. GRUHIER, The IFP Clauspol 1500 process: eight years of industrial experience, Chem. Eng. Monogr., 10 (Large Chem. Plants), 1979, pp.69-86;
HENNICO A., BARTHEL Y., BENAYOUN D., DEZAEL C., Clauspol 300: the new IFP TGT process, For presentation at AIChE Summer National Meeting, Denver (Colo.), Aug. 14-17, 1994.
It is also well-known that the desulfurization rate of a plant of this type can be improved by desaturating the solvent in sulfur according to a process described in patent FR-2,735,460 filed by the applicant. In this case, part of the single-phase solvent and sulfur solution extracted at the end of the contactor reactor is cooled in order to crystallize the sulfur. This crystallized sulfur is then separated from the solvent by various known solid-liquid separation means such as filtration, decantation or centrifugation. A sulfur-depleted solvent that can be recycled to the contactor reactor is obtained on the one hand, and a suspension enriched in solid sulfur that can be reheated to melt the sulfur, then sent to a solvent-sulfur liquid-liquid decantation zone where the liquid sulfur is recovered is obtained on the other hand.
Although such methods prove to be very effective, they can however be limited by constraints.
For example, side reactions occur in the contactor reactor, leading to formation of solid by-products, mainly salts such as alkaline or alkaline-earth sulfates or thiosulfates, due for example to the slow degradation of the catalyst. These solids tend to accumulate and to increase in the decantation zone at the interface between the organic solvent and the liquid sulfur, which makes decantation of sulfur difficult.
One way allowing to overcome this problem is described in patent FR-2,735,460, which discloses the possibility of passing a solvent containing such salts through a filter. The salts settle on the filter, and the sulfur-containing solvent is sent to a sulfur-desaturation stage. Such processing of the circulating solvent is however not sufficient to entirely remove any accumulation of these salts at the liquid sulfur-solvent interface, including the liquid sulfur-solvent decantation zone situated downstream from the zone intended for sulfur desaturation of the solvent.